When framed data is exchanged between a transmission apparatus and a receiving apparatus, it is necessary to establish frame synchronization of a received signal in the receiving apparatus. In a correlation method widely used to establish synchronization in general, when an information signal is sent from the transmission apparatus to the receiving apparatus, a reference signal is added to a head of the information signal. The reference signal is a signal sequence having a predetermined signal pattern. The receiving apparatus calculates a cross correlation value between the received signal and the signal pattern of the reference signal which is known. By detecting timing of a peak of the calculated cross correlation value, start timing of the information signal is specified.
For example, in Bluetooth (registered trademark) Low Energy (hereinafter called BLE), both a reference signal of a preamble and a reference signal of an access address are added to a head of a communication signal that is transmitted. The preamble is a signal pattern in which “0” and “1” are repeated alternately, and is used to detect timing in bit (symbol) units. The access address (synchronization word) is a random value assigned for each connection between the transmission apparatus and the receiving apparatus, and is used to detect timing in frame units.
Usually, when a bit length of the access address is long, in order to suppress delay of processes, instead of detecting a maximum value of the cross correlation value of all bits, a point at which the cross correlation value between the access address and the known reference signal exceeds a detection threshold has been regarded as a frame synchronization point.
However, for example, in a case of a frame format which is periodic and has strong similar pattern nature such as LE Coded PHY in BLE5.0, there exist a plurality of points (called side lobes) with a large correlation value other than the original frame synchronization point (called a main lobe) due to the pattern nature. Therefore, even if a frame detection threshold is set to be low in order to respond to a signal which is in such a state with a small carrier noise ratio (hereinafter called a C/N ratio) and much noise, there is a problem that a frame synchronization point is erroneously detected. Because under a communication signal which is in a state with a large C/N ratio and low noise, an access address cross correlation value in a certain side lobe may exceed the detection threshold before the synchronization point which is a true synchronization point (or a correct point).